Basic power generation technologies are generally grouped according to the energy source used to produce electricity. Fossil fuels such as coal, gas and oil are used to produce steam which is expanded through a steam turbine which, in turn, drives a generator thereby producing electric power. Fuels can also be combusted as in a gas turbine, where the primary energy source is hot gas which again expands and drives a generator. Nuclear power also uses a steam turbine-generator to convert steam produced by a nuclear reactor into power. In the case of geothermal power generation, steam naturally produced by the earth is extracted and processed to an extent, for expansion again, in a steam turbine-generator, although at much lower temperatures and pressures than the aforementioned fossil fuels. While the efficiencies associated with the geothermal steam are much lower than that of the traditional fossil fuels, the steam is essentially free, after the installed cost of the delivery infrastructure, compared to the cost of fossil fuel necessary to produce like amounts of steam. Solar power has also been used to boil water for steam as in Solar One, a plant near Dagget, Calif.
Technologies such as hydroelectric generation utilize the extraction of potential energy from water moved from higher elevations to lower elevations, using the rush of falling water through a "Francis" or "Kaplan" impulse turbine in order to turn a generator and produce electricity. There is no need to produce steam in such a system. The impinging force of the water acting on the water turbine provides the energy to be extracted.
While naturally occurring energy sources such as sunlight, or water are "free" they can vary in supply. In dry years, less hydroelectric generation is available. On cloudy days, less solar power can be generated. Where wind turbines are concerned, at least a mean wind velocity of 10 mph is required to justify installation, because if there is no wind, power is not produced. Similarly, geothermal fields finally expend their available steam, rendering the massive distribution system and generating equipment installed above the field useless. Utility companies and power associations have traditionally attempted to manage such systems: placing hydroelectric systems proximate to predictable watersheds and by building reservoirs; installing arrays of wind turbines in established zones of plentiful and predictable currents; building solar plants in desert locations etc.
Today, in an effort to increase generation thermal efficiencies, technologies are sometimes combined. The best and primary example of such a combination is steam and gas turbine technology. In such a system, a gas turbine is used to generate electricity, and concurrently, the exhaust gases, at nearly 950.degree. F., are directed through a heat recovery boiler to produce steam which is then expanded through a traditional steam turbine-generator. This combination dramatically increases the overall thermal efficiency beyond that seen with either gas or steam technology separately.
The aforementioned combinations are typically not available in the naturally occurring energy resources.
Efforts to find other renewable energy sources to reduce dependence on fossil fuels have spawned alternate fuels including the burning of agricultural waste such as wood chips, almond shells and rice hulls to generate power. Used tires, municipal solid waste in the form of a screened mass or refuse-derived fuel have also provided fuel for power generation. In the case of municipal solid waste, the fuel has been exploited in large part to reduce the amount of waste sent to landfills. To say that the utilization of municipal wastes in the generation of electric power advances the common good would be an extreme understatement.
What is continually needed, then, are ways to extend or augment the availability of renewable or natural resources beyond traditional system efficiency improvements, in order to prolong available energy resources and reduce the dependency on fossil fuels. In conjunction, new methods of utilizing municipal waste and its byproducts are also necessary to ease the environmental impact of simple disposal, and to provide a cleaner environment.
In Sonoma County, Calif., the world's largest geothermal power generation project has been operating for decades. The geothermal field, called The Geysers, was developed by major oil companies, and the giant power generation utility Pacific Gas & Electric Company exploited the field for electric generation, installing several turbine generators, and leasing the resource field from the original developers. Other smaller utility companies have also leased portions of the field for production of electric power. Up to twenty-one units were installed over the years.
In the past decade, the pressure and volume of geothermal energy available in the Geysers field has lessened continually. Pacific Gas & Electric has closed several of the existing units and has curtailed production of others. Plans to retire existing units have been accelerated and staff has been reduced.
In the neighboring community of Santa Rosa, Sebastopol, Rohnert Park and Cotati, millions of gallons of effluent are produced in the local waste water treatment plant. Approximately 30 million gallons per day of effluent are produced in relatively close proximity to the Geysers.
The introduction of 30 million gallons per day of effluent would, over time, replenish the depleted steam resource of the Geysers. The infrastructure necessary to deliver this water to the Geysers will require a pipeline whose capital cost is not unlike that necessary to construct a penstock and/or dam for hydroelectric plants.